This invention relates to methods and apparatus for using coal and natural gas to displace residual oil in existing steam boilers. The methods take advantage of the desirable characteristics of natural gas (including low pollution and high power density) to counteract or neutralize the repulsive characteristics of coal (including high pollution and low power density) to achieve the generally acceptable characteristics of residual oil (middle level of pollution and middle power density) as well as providing improved results as hereinafter more fully described.
The specific characteristics and constructional details of the gas-coal burning apparatus will require various embodiments depending upon the detailed characteristics of the coal used, and the specific oil boiler to be retrofitted. However, three broad classes of gas-coal (oil displaced) burners may be identified as follows:
Type 1 burners, to utilize coal with low sulfur content and low ash content, with high fusion temperature.
Type 2 burners, to utilize coal with low to moderate sulfur content and high ash content, with high fusion temperature.
Type 3 burners, to utilize coal with lower to moderate sulfur and high ash content, with low fusion temperature.
Before describing specific characteristics of each of these types, the general principles and underlying problems in using gas and coal to displace oil will be considered.
Coal, a solid, is an undesirable and even a repulsive fuel. The technology of coal burning is very complex, requiring elaborate facilities for transportation and storage, for grinding and pulverizing, and delivery to the burner, for environmental control of bottom ash, fly ash and other suspended particulates, and for SO.sub.x and NO.sub.x suppression. For the United States and in some other countries, coal has one very important and redeeming feature, namely the abundance thereof.
Oil, a liquid hydrocarbon, is a more acceptable and neutral fuel. Handling and storage problems are somewhat simpler than coal. However, the predominant grade of oil used in steam boilers, No. 6 or residual oil, also has associated SO.sub.x and NO.sub.x, ash and heavy metal problems. More importantly, however, is the fact that the United States can barely produce sufficient oil to satisfy its needs for transportation, where the liquid form is more essential at this time. The problems are even more compounded in other countries where no oil is produced. The use of oil by utilities and industry must be viewed as a major cause of our excessive dependence upon imported oil, and the associated severe economic problems the United States and most of the world are now facing.
Natural gas, a gaseous fuel consisting primarily of methane (CH.sub.4), is a highly attractive fuel. Transportation by pipeline and handling are very simple and local storage is not required, if an adequate pipeline is provided. The burning system for natural gas is simple and there are no ash and SO.sub.x problems. Unfortunately, there are regulatory problems associated with natural gas which stem from pre 1978 estimates of natural gas reserves which suggested that United States production would decline from 20 TCF (Trillion Cubic Feet) to about 10 TCF by the year 2000. Based upon such an assumption, The Public Utility and Industrial Fuel Use Act (PIFUA) of 1978 was enacted to phase out the industrial and public utilities use of oil and natural gas. However, recent drillings, while supporting declining oil reserve projections, have not supported declining natural gas projections. Rather conservative projections now favor a continuous annual production capability to the year 2000 at about 20 TCF, and optimistic projections suggest 30 TCF might be achieved. In the present analysis the middle of the road projection will be used, namely that natural gas can provide approximately 25% of the energy needs of the United States through the year 2000 AD.
Granting the abundant availability of coal, a fuel with repulsive characteristics (high pollution, lower power density) the moderate availability of natural gas, with attractive characteristics (low pollution, high power density) a novel solution emerges to the serious problems posed by the shortage of oil in the United States, namely a fuel with neutral characteristics (accepted level of pollution, middle power density). In essence the invention provides methods and apparatus in which coal is burned simultaneously with natural gas in existing oil boilers or in newly designed coal and gas boilers. The basic concept is to use the attractive features of gas to neutralize some of the repulsive features of coal thereby to achieve the more neutral qualities equivalent or better than that of oil. Specifically the coal-gas burning apparatus and methods will replace residual oil burners, since residual oil is most widely used in steam generation, as well as replacing other types of oil burners.
Successful exploratory tests of burning various mixtures of gas and coal together indicate that gas and coal can be burned together in a variety of arrangements. Among the quantitative considerations favoring a combination of gas and coal to replace oil in oil boilers is the power density consideration. From the dimensions of existing boilers with the same power ratings, it is known, that the normal power densities achieved in typical gas:oil:coal burners vary as about 1.33:1:0.72. This suggests that, gas and coal burning in about equal proportions by energy could be carried out at the normal power density of oil burning.
Residual fuel oil typically yields about 18,000 BTU/lb, whereas coal yields about 14,000 BTU/lb and methane 24,000 BTU/lb. Thus, from a simple energetics standpoint, it would be expected that 60% by weight of coal and 40% by weight of natural gas would lead to the 18,000 BTU/lb of No. 6 fuel oil.
Another simple quantitative consideration, favoring use of gas and coal to replace oil, is based upon the fact that the hydrogen to carbon weight ratio of No. 6 fuel oil is approximately 1 to 9. Granting coal to be predominately Carbon (12) and CH.sub.4 to be 4/16=1/4 hydrogen and 12/16=3/4 carbon by weight, it should be clear that a burning proportion of 2 carbon atoms to 1 methane molecule, about 60% by weight of coal; and 40% by weight of gas will provide the hydrogen:carbon weight ratio in residual fuel oil.
From the sulfur dioxide pollution standpoint, a mixture of gas and coal can also simulate oil. Take as a representative example, residual fuel oil which releases 1.5 lbs SO.sub.2 /MBTU (1 MTBU=10.sup.6 BTU), which released 2 lbs SO.sub.2 /MBTU and natural gas which releases negligible SO.sub.2. To maintain the oil emission with gas and coal simply requires burning the two fuels in the proportions 25% and 75% (gas to coal). Other examples can be determined as may be required. It should be noted that the SO.sub.2 output emission is twice the weight of the sulfur output.
The low ash content of oil (typically 0.1%) is one feature which is not easily replaceable by a mixture of gas (which has no ash) and coal. Thus, it is not possible, with the use of reasonable proportions to reduce the ash production of a gas-coal mixture to the ash production of oil. Furtunately, however, precipitators do capture fly ash in the flue emissions with very high efficiency (99%). Precipitators, however, are expensive and boilers have problems with large quantities of ash. Therefore, when the coal used has a high ash content, it is useful to remove as much ash, as possible before injection into the boilers. Here the invention, as hereinafter described, can be employed for pre-boiler ash removal.
The description provides the main considerations entering the design of three types of gas-coal burners to displace oil burners in boilers. In all the embodiments of the invention, the gas-coal burning mixture is used to simulate the overall power density, energy release/lb and carbon-hydrogen weight ratio of an oil flame, and to approach or decrease the SO.sub.2 source density (e.g. lbs of SO.sub.2 per million BTU energy release) of the oil burner and boiler.
In addition to the foregoing principles, additional methods and constructional details of improved burning apparatus achieve higher power densities when there is a shortfall in the gas-coal burning power density, with respect to the desired power density or for other purposes. These methods have been suggested in general in many early patents and they include means for imparting, swirling, spiraling, or turbulent motion to fuel-air streams. The prior art U.S. Pat. Nos. 1,947,866--McCourt; 2,335,188--Kennedy; 3,049,085--Musat et al; 3,163,203--Ihlenfield; 3,716,324--Ponthroreau et al; 4,003,692--Moore; and 4,094,625--Wang. The most pertinent of the prior art references are believed to be Zinn (U.S. Pat. No. 3,302,596) and Kennedy (U.S. Pat. No. 2,335,188) neither of which teach or disclose the invention as claimed herein.
A few definitions will be of assistance in understanding the invention herein:
(A) Trochoidal motion refers to curved motion in a plane, such as the curved traced by a point on a circle, which rolls on a straight line or on the inside or outside of another circle. Such motion is useful, for example, to slow down the effective propagation of a flame front when the fuel air mixture is delivered at velocities exceeding the normal flame propagation velocity.
(B) Helical motion or spiral motion, refers to motion in three dimensions, such as that of a point on the thread of a screw as it is twisted into a nut. The path the point travels is like that of a coiled spring. Such motion is useful to increase the time of transit, for example, of a burning particle moving or shot from one side of a burner or boiler towards the opposite side.
(C) Turbulent motion is frequently imparted to fuel and air in burners. This motion may be considered as a random combination of trochoidal and helical motion, and is frequently used in burners to promote good mixing of the fuel with air.
To illustrate the intended objective, consider a representative situation for retrofitting an existing oil boiler to employ the invention. A conversion to coal alone would require a derating of about 60% of the actual oil boiler capacity. With the present principles and techniques for gas-coal burning, it would appear possible by burning gas and coal in 25% to 75% proportions to increase the actual capacity above coal alone by 20%. Higher power levels are achievable by stepping the gas supply upward to 40% of rated capacity while maintaining the coal power at 60%. Such a retrofit would utilize greater reliance upon coal than has been possible in coal-oil mixtures (COM), another proposed approach to the conversion of oil boilers. The best known experience on COM conversion uses coal energy only to the extent of 40% of the mixture. Thus, the COM approach reduces total reliance on residual oil only by 40% but takes on all the undesirable features of a coal conversion, and usually compounds the pollution problems, etc.
The basic principles underlying the design and objectives of gas-coal (oil displaced) burners, the three types of gas-coal burner-boiler systems incorporating these basic principles, as well as other innovations, will be further described hereinafter.